1. Field of the Invention
The present invention relates to a fixing apparatus used for an image forming apparatus such as an electrophotographic printer and a copying machine, and a film for use in the fixing apparatus.
2. Description of the Related Art
Fixing apparatuses including a film heating system are available as a fixing apparatus used for an image forming apparatus including an electrophotographic system.
The fixing apparatus including the film heating system includes a tubular film with high heat resistance, a ceramic heater (hereinafter described as a heater) in contact with an inner surface of a film, and a pressure roller that forms a nip portion with the heater via the film. The fixing apparatus fixes a toner image on a recording material, while conveying the recording material bearing the toner image, at the nip portion. The heater and the film for use in the fixing apparatus including the film heating system have a low thermal capacity, and thus have an advantage that an electric power can be saved and a waiting time can be shortened (a quick start can be realized).
In the above fixing apparatus, when a high resistance fluorine resin layer is used as a surface layer of the pressure roller in order to prevent a paper dust and a toner from adhering to the surface of the pressure roller, the pressure roller is sometimes charged with a charge having the same polarity as that of the toner by sliding friction with the recording material. As a result, there has been a problem that an offset (hereinafter described as an overall offset) occurs in which an unfixed toner borne by the recording material is peeled off by an electrostatic force, adheres onto the film at the nip portion, and appears as an image on a subsequent round.
As a countermeasure to the overall offset, a technique in which a voltage having the same polarity as that of the toner is applied to the film to form an electric field in a direction where the toner is adsorbed to a recording material has been discussed in Japanese Patent Application Laid-Open No. 9-80946.
There is another problem as a separating offset in addition to the aforementioned overall offset. The separating offset occurs when a film surface is strongly charged locally with a charge having a polarity reverse to that of the toner by a separating discharge generated when a trailing edge of the recording material passes through the nip portion, thereby forming an offset electric field when the charged area faces the recording material.
As a countermeasure to this separating offset, a technique in which a resistance value of a fixing member (film) is lowered to reduce the charge has been discussed in Japanese Patent No. 3397548.
To practice the countermeasure to the overall offset in Japanese Patent Application Laid-Open No. 9-80946, it is necessary to increase the resistance value per unit area between an outer surface and an inner surface of the film so as to make a current harder to flow in the film and keep a surface potential on the film when the voltage is applied to the surface of the film to have a potential having the same polarity as that of the toner. However, although the overall offset can be prevented when the resistance value of the film is increased, the separating offset becomes worse because the separating charge on the film formed when the recording material is separated does not sufficiently attenuate.
Meanwhile, to practice the countermeasure to the separating offset in Japanese Patent No. 3397548, it is necessary to reduce the resistance value of the film so that the charge charged on the outer surface of the film quickly attenuates. However, although the separating offset can be prevented when the resistance value per unit area between the outer surface and the inner surface of the film is reduced, the overall offset becomes worse because the current flows from the film toward the pressure roller by the voltage applied to the film.
As described above, it is difficult to balance the countermeasure to the overall offset with the countermeasure to the separating offset.